The Mars atmosphere is thin and composed mostly of carbon dioxide. The gravity is lower than the gravity on Earth. More specifically, atmospheric density near the Martian surface is roughly equivalent to an altitude of 100,000 feet on Earth. The low atmospheric density leads to a high cruise velocity, but a low Reynolds number. Since the speed of sound on Mars is lower than on Earth, transonic aerodynamic effects are encountered at a lower flight speed. Low atmospheric density also presents difficulty in generating sufficient thrust. Therefore, with such a thin atmosphere, generating enough lift to support airplane weight is difficult and wing loading must be small.
In addition to the challenge of generating sufficient lift and thrust to fly an airplane through the Martian atmosphere, there are challenges associated with getting the airplane to Mars. These challenges include stowage of the airplane in a non-flight configuration and the extraction of the airplane from the non-flight configuration to an in-flight configuration capable of flight in a thin atmosphere.